Co-Head of the United Standings Improvement Agency [USIA] here.
This is behavior CCP hasn’t explicitly stated or documented, so your experience fills a gap of our collective understanding. Normally the higher of your corp or faction standings is used to determine diamond mining rat behavior. If you’ve never pissed them off (never fired at them, never missioned for their corps and repeatedly failed or repeatedly declined missions from them, etc), then mildly negative faction standings would not provoke them to attack. Clearly, passing the Kill-on-Sight threshold takes precedence over NPC corp standings when determining their behavior. If I had to guess, I would anticipate this would be the case even if you had very high standings with the NPC mining corps but still had KOS with the faction itself.
I also want to point out that NPC diamond mining rats from multiple corps of multiple factions can mine any region of space, even any one single system, at any given time. In my home system over in Minmater HS, it is not uncommon for D-scan to reveal diamond mining rats from a Minmatar, Amarr, Angel, Nefantar, and/or Thukker corps all in system at the same time, so you might be liable to be attacked by one KOS faction if not another at any given moment. Note that secondary empires and pirate factions were present in the primary empire space. You might want to consider repairing faction standings if you do not want to be inconvenienced by such encounters.
DMC’s Faction Standing Repair Plan is a good place to start with faction standing improvement, and we over here at USIA have developed additional methods not covered in the plan (6x L4 storylines in one day for faction boosting is lovely), along with a protocol for raising and maintaining the standings of opposing factions simultaneously (yes, it is possible for all factions to be simultaneously positive, I’ve mathed it out). We have a guide on our website right now, An Extension to The Faction Standings Repair & Improvement Plan: Simultaneously Improving and Maintaining Standings with Opposing Factions via Storyline Blitzing, though I’m planning on adding A LOT more content and breaking it down into two or three smaller guides (with shorter titles!!!). Until the updates are published, you are welcome to join the USIA Discord server for advice on raising your faction standings so we can share with you anything that hasn’t been covered in DMC’s guide or hasn’t yet been added to our guide.